Diversity analysis of type I ketosynthase in rhizosphere soil of cucumber

Fusarium wilt [Fusarium oxysporum (Sch1.) f.sp. cucumerinum Owen.] is a major soil-borne disease of cucumber worldwide, and can cause huge yield losses. Biological control of Fusarium wilt of cucumber has received considerable attention. Many bacteria, particularly actinomycetes, are known to produce secondary metabolites synthesized by Polyketide synthases (PKSs) with a diverse range of biological activities. Ketosynthase (KS) gene diversity was analyzed in samples which were collected from rhizosphere soil of both diseased cucumber and healthy cucumber in Dalian, China. The phylogenetic analysis amino acid (AA) sequences indicated that the KS genes in the rhizosphere soil samples were clustered into diverse seven clades, including Sorangium cellulosum, Anabaena variabilis, Nostoc punctiforme, Xanthobacter autotrophicus, Streptomyces, myxobacteria and uncultured bacteria. Among seven major clades in the phylogenetic tree, two clades were peculiar to rhizosphere soil of diseased cucumber and one was peculiar to healthy cucumber. Among the 182 cloned KS genes, 147 KS genes were clustered with the uncultured bacteria group. Most of the KS genes showed about 80% similarity at the AA level to sequences known in GenBank. These results revealed the great diversity and novelty of KS genes in rhizosphere soil of cucumber.